Transcript Lecture 3

Quiz Preparation
Have Quiz sheet ready.
 Top Center: QUIZ 3
 Upper Right:
» Name (L, F, MI),
» Today’s date 9/4/12
» Lab day , time
» section number

Lab Day/Time
Mon.
Mon.
Wed.
Tue.
Wed.
Fri.
Wed.
Thu.
Thu.
Fri.
Fri.
1:00
3:00
1:00
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1:00
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Section
Number
8
9
10
36
10
11
12
11
29
35
12
12
1
Announcements

Read Chapter 6 before lab.

Work problems at the end of the Chapter 6 for
practice. Solutions on web site.

Be sure to do the Pre-Lab assignment. You will
lose in-lab points if you do not have the pre-lab
worked when you arrive at lab.
2
Electricity – Water Analogy

Electricity flows through a circuit like water
through a pipe.
» charge -- # of water molecules in the pipe
» current -- water flow rate (gallons per minute)
» voltage – pressure (psi = pounds per square
inch)
» resistance -- resistance to flow (inverse of pipe
cross-sectional area)
» capacitance -- water stored under pressure (e.g.
a bladder tank)
3
Units
charge -- coulombs (C)
 current -- amperes (A), A=C/s
 voltage -- volts (V)
 resistance -- ohms (),  =V/A
 capacitance -- farads (F), F = C/V
Note: Farads are huge units -- usually use F
(microfarads)

4
Circuit Elements

voltage source (e.g. battery) – like water
pump with specified pressure. Units: volts
(V)
5
Circuit Elements

resistor -- pipe with small cross-sectional
area that impedes water flow. Units: ohms
()
Wire can be viewed as a resistor with very low or zero resistance.
6
Circuit Elements

CAPACITOR – a reservoir or sponge that
can store water (charge). Units of
capacitance: Farads (F)
7
Circuit Elements

diode -- valve that only allows current flow
one direction. (No units)
current
8
Quiz

1. Electric current is analogous to:
» a. water pressure
» b. water flow rate
» c. # of water molecules in the pipe

2. Voltage is analogous to:
» a. water pressure
» b. water flow rate
» c. # of water molecules in the pipe
9
Ohm’s Law
The current I flowing through a resistor is
proportional to the voltage V across it and
inversely proportional to the resistance R:
V
I
or
R

V  IR
10
Ohm’s Law - Example
I
100 
+
Always:
Label
5V
current
directions
-
Put
+/- signs on
voltages
I = 5/100 = 0.05A
Passive sign convention: place the positive voltage
reference at the same terminal that the current enters
11
Quiz
3.
The current I is
(a) 100 A
(b) 0.01 A
(c) -10,000 A
I
1000 
+
10V
-
12
Connection Types

Series (daisy chain)

Parallel (side by side)
13
Series Connection

Single branch. Current is the same through
all elements.
I
14
Series Connection

Resistors add in series.
Ex: R1=50, R2=100   R=150 
15
Multiple Voltage Sources

Multiple voltage
sources in series add.

These circuits are
equivalent.
16
Wait – I thought they ADDED!
10 + 4 = 6?
 Have to look at the polarity

+
10
-
1k
4
+
17
Parallel Connection

Multiple branches connected at both ends.
The voltage is the same across all elements.
18
Parallel Connection

The reciprocal of resistance adds.
» 1 1 1
R
R1
R2

R
1
1 1

R1 R2
For two resistors
only = product /
sum
Ex: R1=50, R2=100   R=(50)(100)/150=33.33 
19
Series vs. Parallel
Are
the lights in your house/apartment
wired in series or parallel?
Answer: Parallel!
20
Voltage Division
(wire)
(motor)
R2
V2
V1
R1  R 2
• If R1 represents the wire resistance and R2 represents the
motor and motor circuit resistance, we want R1 to be as small
as possible. This will deliver the maximum voltage to the
motor.
21
Voltage Division - Example
10
15V
20
20
V2
 15  10V
10  20
22
Resistors in Parallel - Example
I1
I2
Vo

What is the current in each branch?
23
Resistors in Parallel - Example
I1
I2
Vo

Voltage is the same across each branch– use
Ohm’s law:
V0
I1 
R1
V0
I2 
R2
24
Resistors in Parallel - Example
I1
Vo
15V

I2
10
20
Ohm’s law
15
I1 
 1.5 A
10
15
I2 
 0.75 A
20
25
Series/Parallel Example
I1

I2
What is the current in each branch?
26
Series/Parallel Example
I1
I2
2k

Combine the two 1kΩ resistors in series.
27
Diode Example
+ VR Vo

R
I
What is the current in the circuit (I) and the
voltage drop across the resistor (VR)?
28
Diode Example
+ VR R
Vo
I
I
VD=0.7V
Vo  VD 


R
VR  IR
29
Diode Example
+ VR 9V
I
9  0.7 


1k I
VD=0.7V
 0.0083 A  8.3mA
1000
VR  (0.0083)(1000)  8.3V
30
Breadboards

Used for rapid prototyping
» Easier & faster than soldering
» Allow for easy alterations
Generally for temporary use
 Components just plug in

31
How a Breadboard Works
Connect
components by
plugging them in
 Components are
connected by
copper wires
underneath holes

32
Connecting Resistors in Series
33
Connecting Resistors in Parallel
34
Quiz
4. Voltage division is used
(a) to find the voltage across a single element in a
series circuit
(b) to find the voltage across a single branch in a
parallel circuit
(c) to determine the fair allocation of batteries in a
team-based design
35
Quiz
5. Current division is used
(a) to find the current through a single element in a
series circuit
(b) to find the current through a single branch in a
parallel circuit
(c) to determine the fair allocation of batteries in a
team-based design
36